Powder-air venturi for electrostatic spray coating system

ABSTRACT

In an electrostatic powder spray system wherein resin powder is fed from a supply hopper to a powder dispensing venturi and is propelled therefrom by pressurized air through a conduit to a spray gun, the venturi housing is constructed of insulating material so that resin powder particles electrostatically charged by the triboelectric effect are not attracted to the venturi housing walls and do not clog the venturi.

DUMP VALVE LEVEL comm Unlted States Patent 1 1 1111 3,724,755 Diamond at al. 1451 Apr. 3, 1973 1541 POWDER-AIR VENTURI FOR 3,358,639 12/1967 Lopenski et a1 ..239 15 x ELECTROSTATIC SPRAY COATING 3,405,291 l0/l968 Brandmaier.... ..239/15 X 1 3,630,442 12/1971 Probst ..239/l5 SYSTEM 3,554,445 1/1971 Engwall... ..239/3 X [75] Inventors: Harold G. Diamond, Wexford; Floyd 3,591,080 7/1971 Kock ..239/3 x L. Gustine, Pittsburgh, both of Pa. [73] Assignee: Allis-Chalmers Corporation, Mil- Primary Examiner-Richard A- Schacher waukee, Attorney-Lee H. Kaiser et al.

[22] Filed: Sept. 27, 1971 57 ABSTRACT [21] Appl' 183,939 7 In an electrostatic powder spray system wherein resin Related Application Data powder is fed from a supply hopper to a powder dispensing venturi and is propelled therefrom by pres- 1 fggg g g- -g 3 F 1 19 2P? S surized air through a conduit to a spray gun, the vena one a con mualon'm'pa 0 turi housing is'constructed of insulating material so z fifi g g :3 f fizg g i 3 that resin powder particles electrostatically charged by I I the triboelectric effect are not attracted to the venturi 52 US. (:1. ..239/1s housing Walls and the venturi- 51 Int. Cl. ..B05b 5/02 [58] Field of Search ..239/3, 15

[56] References Cited 4 Claims, 11 Drawing Figures UNITED STATES PATENTS 3,493,540 3/1970 Adams ..239/15 PATENTED APR 3 I975 sum 1 or 4 l 25 AIR ExHAusr "3i 24 I" Q POWDER SEPARATOR we? a \\J VALVE SPRAY BOOTH PATENTEDAFR 3 I975 SHEET l 0F 4 MOTOR CONTROL PARJELES CHARGED BY TRIBOELECTR\C EFFECT POWDER-AIR VENTURI FOR ELECTROSTATIC SPRAY COATING SYSTEM This application is a continuation-in-part of our parent application Ser. No. 722,078, entitled Electrostatic Resin Powder Spray System," filed April 17, 1968, now abandoned; of our continuation-in-part ap plication Ser. No. 728,253, filed May 10, 1968 having the same title, now abandoned; and also of our copending application Ser. No. 41,229 filed May 28, 1970, entitled Electrostatic Resin Powder Spray System Having Improved Powder Dispensing Means.

This invention relates to apparatus for coating metallic articles with a resinous material.

The metallic covers of electrical apparatus such as distribution transformers are often covered with a relatively thick resin coating to prevent animals and birds coming in contact with the cover from being electrocuted. The resin is conventionally a combination of specific materials such as epoxy resins and polyamide resins and is usually applied by a fluidized bed process in which the metallic article to be coated is heated to a temperature above the melting point of the resin and immersed into a fluidized powdered resin mass to cause the resin particles to melt and adhere to the heated metallic article and to flow into a smooth uniform resin coating on the article. I

Resin powder particles may vary in size between 0.2 to 500 microns. The 0.2 micron sizes are easily airborne, andat constant air velocity they are quickly removed from the fluidized bed, leaving increasing amounts of coarse fractions behind. When the major amount left is coarse and cannot be airborne without greatly increasing the air velocity, volcanoes and blowholes occur which result in uneven coating and streaking of the resin coating on the metal article. The curing cycle forflowing out the powder into a uniform coating is sensitive to the presence of a sufficient volume of fine resin particles to initiate melting at a low constant temperature, and the imbalance of particle sizes may change the curing cycle and necessitate scrapping of large amounts of resin powder.

This fluid flow separation of particle sizes is known as elutriation. It is a disadvantage'in fluidized bed operations.-Elutriation is avoided in our apparatus by means for keeping the air flow and velocity constant for a given empirically determined small volume of mixed size particles so that all of the sizes are airborne and expelled through the spray gun before elutriative separation can occur. The overspray, which contains a higher proportion of course particles than virgin powder, is also kept to small increments for blending back into virgin powder so as to maintain a total volume for electrostatic spraying that contains a size distribution that powder is introduced into a container 'such as a fluidized bed where it is mixed with new powder, and the resulting mixture is then fed into hoses leading to the spray nozzles. In addition to the fine particles settling in such fluidized bed, it was found that the recovered overspray contains agglomerated resin particles which tend to accumulate near the bottom of the bed. Such accumulation .of agglomerated particles often grows until the entire contents of the bed, which may contain several hundred pounds of powder, must be scrapped.

Conventional electrostatic spray resin coating systems have numerous shutdowns and require the continued presence of an operator. Such shutdowns are usually caused by obstructions resulting from compaction of the powder which block circulation of the resin particles, and the expense of the continuous presence of an operator and the downtime of the spray coating apparatus substantially increases the cost of resin coating. Further the percentage of resin powder particles which are actually used in coating the metal article is relatively low in conventional systems, and the air surrounding such conventional spray systems is usually contaminated with the resin powder particles.

It is impractical in such an electrostatic resin spray coating system to shut off the powder, and the powder spray is controlled by shutting off the pressurized air. The resin powder is usually fed by gravity to the injection point in a powder-air venturi chamber, or powder suppy nozzle chamber and is carried away during the time 'that the air flows through the central jet in the venturi. When the pressurized air is shut off, the resin powder continues to flow by gravity and fills the powder-air venturi chamber and passages connected thereto. The extremely fine resin particles appear to continue to move when the compression air is shut off and to compact on the walls of prior art powder supply nozzles to the extend that they block the passageway for the powder after only a few days of operation. The extremely fine resin particles find their way into the screw threads behind the pressurized air supply nozzle, or ram which controls the amount of powder mixed with the air, so that they jam the threads and prevent adjustment of the ram. The resin particles accumulate and adhere to the walls of prior art powder supply nozzles to such an extent that it is impossible to force sufficient'powder through the venturi for spray coating even when the air pressure is raised from the normal operating range of l520 pounds per square inch to pounds per square inch, and the powder adheres so tenaciously to the walls of known powder supply nozzles that a cutting tool is required to remove the compacted particles.

We have found that an electrostatic spray resin coating system which utilizes a large volume of resin powder has many more shutdowns resulting from compacted powder obstructions than one which uses a small volume of powder; that the overspray contains a much higher percentage of coarse resin particles, i.e., of'approximately mesh size, than the new powder; that such overspray resin particles tend to clog the conventional powder separator and the main supply hopper which feeds the spray nozzles if a large volume is permitted to accumulate even when the overspray powder is mixed with the new powder; that a high percentage of such coarse overspray particles in the powder fed to the spray nozzles changes the curing cycle of the resin and results in streaking and inferior coatings; that such coarse overspray particles settle and accumulate near the bottom of a fluidized bed used to feed the spray nozzles and result in feeding powder to the nozzles which does not have a uniform distribution of particle sizes and may necessitate scrapping of the entire bed; that the resin coating produced in a system having a fluidized bed feed for the electrostatic spray nozzles often streaks and contains voids which make it unsatisfactory for applications where the coating must have high dielectric strength; and that an optimum resin coating without voids is obtained when a uniform distribution of powder particles sizes is fed to the nozzles which results in a coating having the closest possible packing of powder particles.

It is an object of the invention to provide an improved electrostatic spray resin powder coating system and method which prevents accumulation and adherence of the resin powders on the powder-air venturis which control the flow of powder to the spray nozzles and eliminates clogging of such venturis.

This and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is an elevation view, partially schematic, of an electrostatic spray resin powder coating system embodying our invention;

FIG. 2 is a side elevation view of the powder dispensing means of the system of FIG. 1;

FIGS. 3 and 4 are front and side elevation views respectively of the dump valve of the apparatus of FIGS. 1 and 2 with portions broken away to better illustrate the internal construction;

FIG. 5 is a front elevation view of the improved venturi means of the invention;

FIG. 6 is a section view through the venturi shown in FIG. 5;

FIG. 7 is an elevation view, partly schematic, of an alternative embodiment of electrostatic spray resin coating system incorporating our invention;

FIG. 8 is a schematic plan view of the powder separator of the system of FIG. 7;

FIG. 9 is an elevation view, partly schematic, of another alternative embodiment;

FIG. 10 is an enlarged partial view taken on line X- X ofFIG. 9; and

FIG. 11 is a partial view through the powder-air venturi housing of the invention illustrating the region wherein the resin particles are electrostatically charged by the triboelectric effect.

Referring to FIG. 1, the metallic article, or workpiece to be coated with resin is shown as a distribution transformer cover 10 which has been heated above the melting temperature of the resin to be applied and is being moved through a spray booth 11 on an overhead conveyor including an I-beam l2, movable support means 13 having wheels 14 engaging the lower flange of the I-beam l2, and hook means 15 depending from the movable support means and engaging the cover 10. The workpiece 10 is schematically shown to be grounded, and the resin particles accumulate an electrostatic charge as they pass through the spray nozzles 17, which are connected to a suitable high potential source shown schematically as a battery 19, and are attracted to the grounded workpiece 10. The resin particles which do not contact the workpiece 10 are withdrawn through ducts 21 from the upper and lower ends of spray booth ll vertically upward into a powder separator 23 by suitable air circulating means shown schematically as a fan 24 disposed at the entrance to an air exhaust duct 25 on the housing of separator 23. The recovered powder particles are returned by gravity from separator 23 through a duct 28 to a recirculated powder screening bin 30 where they pass through a grate type hopper magnet 32 which attracts any ferromagnetic particles therein. The recovered resin particles fall onto an inclined vibrating screen 33 actuated by an electromagnet 35, and any agglomerated resin particles or debris which do not pass through the screen 33 slide down the vibrating screen and fall into a slight glass container 37.

The recirculated resin powder particles which pass through screen 33 fall into a generally cone-shaped powder supply hopper 39. The walls defining hopper 39 are relatively flexible and are inclined at a steep slope and may have steel plates 40 welded thereto which are cyclically attracted by electromagnetic vibrators 42 to flex and vibrate the hopper walls and thus cause the powder to fall into the bottom of the hopper.

The throat at the lower end of a new powder hopper 44 positioned adjacent the recirculated powder bin 30 registers with the aperture 45 (see FIG. 4) and the upper end of a motor-operated dump valve 47. Dump valve 47 is cylindrical and has a plurality of elongated powder receiving depressions 49 in its outer periphery which extend parallel to its axis. Dump valve 47 is rotated by an electric motor 52 through a suitable gear speed reducer 54 and belt and pulley means 55. The new resin powder is introduced into new hopper 44 and falls into the elongated powder receiving depressions 49 in dump valve 47, and the amount of new powder delivered through aperture 56 at the lower end of dump valve 47 into powder supply hopper 39 is determined by the rate of rotation of dump valve 47.

A bin level control 57 on a sidewall of powder supply hopper 39 provides an electrical output signal to a dump motor valve control means 59, shown schematically in block form, when the amount of new and recirculated resin powder in supply hopper 39 is below a level indicating that a predetermined amount of powder, preferably approximately five pounds, is in supply hopper 39. In response to the output signal from bin level control 57 valve control means 59 energizes motor 52 to rotate dump valve 47 and introduce new powder into supply hopper 39 until this predetermined amount is reached, at which time bin level control 57 removes the electrical signal to valve motor control means 59 to deenergize motor 52.

The mixture of new and recirculated resin powder in supply hopper 39 falls by gravity through a powder inlet port 58 into a venturi chamber 60 within the housing 61 of each of a plurality of powder-air venturis 62. A suitable supply of pressurized air is connected by conduits 63 to the powder-air venturis 62, and the pressurized air flows through an air inlet passage 64 in each housing 61 and thence through a radial aperture 65 in an adjustable ram, or pressurized air nozzle 66 disposed within an air supply, nozzle receiving compartment in housing 61. Ram 66 has a smaller diameter portion 67 with ()-ring gaskets at each end thereof which permits the air to flow into radial aperture 65 regardless of the ram position and thence through an axial restricted central venturi aperture, or pressurized air orifice 68, in ram 66 which is transverse to the powder inlet port 58 into the chamber 60 to force the resin powder out of chamber 60 through a diverging outlet passage 69 in housing 61 which is aligned with central venturi aperture 68 and into flexible hoses 70 connected to the powder-air venturis and to the electrostatic spray nozzles l7. Pressurized air supply nozzle 66 has external threads 71 which engage internal threads in housing 61 to permit axial movement of the ram 66 into and out of chamber 60, when it is rotated by knurled knob 72, to positions wherein it partially obstructs powder inlet portg58 to thus control the amount of resin powder which mixes with the air.

It is our theory that the stream, of compressed air at the exit from the jet venturi aperture, or pressurized air opening 68 acting upon the resin powder particles falling into chamber 60 function as a triboelectric generator which electrostatically charges the powder particles as schematically illustrated in FIG. 11, thereby causing them to compact on the grounded metal housing of prior art powder-air venturis. The powder particles so charged by the triboelectric effect were'attracted to priorart grounded metal housings and often blocked the outlet passageway after only a few days of operation. When the supply of compressed air was shut off, the extremely fine resin particles were attracted to the grounded metallic pressurized air supply nozzle and the portions of the prior art grounded metallic housing surrounding the ram so that they jammed the threads and prevented adjustment of the ram. i

The new'powder particles preferably vary in size from 40 to 140 microns. The particles larger than a predetermined size, e.g.,-No. 40 mesh, are removed by vibrating screen 33. We believe that the electrostatic attraction to the grounded metallic powder-air venturi housing of prior art apparatus was greater for very fine particles of l to 5 micron size because of their greater surface-to-volume. ratio. The electrostatic charge on each particle is proportional to its surface area which varies as the cube of the diameter, whereas volume varies only as the square of particle diameter. The very small powder particles electrostatically charged by the triboelectric effect at the exit from venturi jet opening 68 were attracted to and adhered so tenaciously to the walls of prior art grounded metallic venturi housing that it was impossible to force sufficient powder through the powder-air venturi 'fof spray coating even when the air pressure was raised from the normal operating range of- -20 psi to 90 psi. The charged powder particles were so attracted to the metallic venturi housing walls that a cutting tool was required to remove the compacted particles.

powder-air venturis 62 constructed of insulating material do not attract the powder particles electrostatically charged by the triboelectric effect at the exit from the venturi jet opening 68 and do not clog even after months of operation. Further, the charged powder particles are not attracted to and do not jam the threads of ram 66 which is isolated from electrical ground.

We believe that all of the powder particles are charged to the same polarity by such triboelectric generator so that they repel each other and do not a'g glommerate as they are blown through conduits 70 to the spray nozzles 17.

We have found that an electrostatic spray resin coating system which utilizes a large volume of resin powder has frequent shutdowns resulting from obstructions caused by compacted powder, and FIG. 7 illustrates an alternative embodiment of our apparatus which permits a smaller amount of powder to be in the system than the apparatus of FIGS. ll-6, the elements similar to those of the system of FIGS. 1-6 being given the same reference numerals. The source of pressurized air is shown as a pump connected to conduit 63 and an adjustable regulating valve 77 which may be adjusted to vary the pressure of air supplied to venturis 62 and thus control the volume of resin powder from the nozzles 17. Means schematically shown in FIG. 7 for minimizing the amount of resin powder in the electrostatic spray resin coating system includes a normally closed valve 79 in conduit 63 operable to open position by anelectrical operating coil 80 to supply pressurized air to venturis 62 when the coil 80 is energized and means responsive to the presence of an article to be coated opposite the spray nozzles 17 for completing an energizing circuit to operating coil 80. The means for completing an energizing circuit to operating coil 80 are schematically shown as a normally open limit switch 81 in series with a source of electrical power 82 and the coil 80, and an operating arm 84 for switch 81 in the Path of movement of the conveyor support means 13 adapted when engaged by support means 13 to operate limit switch 81 to closed position and thus complete the energizing circuit to coil 80 to thereby open valve 79 and supply pressurized air to venturis 62. It will be appreciated that such means will shut off the supply of resin powder when no workpiece 10 is opposite the spray nozzles 17 and will thus minimize the amount of powder recirculated in the system.

The powder separator 23 preferably has a plurality of depending inflatable bags 27 within a housing 86. Each bag27 has a fluid venturi opening 87 at the open upper end thereof, and means are provided to intermittently puff pressurized air into each bag 27 to inflate it and thus cause overspray resin particles that have settled on the bag to fall by gravity into the lowerend of housing 86. The means for intermittently inflating bags 27 schematically shown in FIGS. 7 and 8 includes a source 'of pressurized air such as a pump 90, a manifold 92 receiving pressurized air from pump 90, a plurality of headers 93 each of which communicates with manifold 92 and is disposed above the open end of a plurality of bags 27 and has a plurality of orifices-(not shown) therein each of which registers with the venturi opening 87 at the upper end of one of the bags 27, a normally closed valve 95 between each header 93 and manifold 92, a pilot valve 96 associated with each valve 95 having an operating coil (not shown) and being adapted to actuate the corresponding valve 95 to open position when its operating coil is energized, a rotary electrical switch having a plurality of stationary contacts 98 arranged in a circle with each stationary contact 98 connected by conductor means to the operating coil of one of the pilot valves 96, a movable contact 100 adapted when rotated to sequentially engage the stationary contacts 98 and being connected to one side ofa source of electrical power 102, and means such as a motor 103 for rotating movable contact 100. It will be appreciated that when movable contact 100 rotates, the pilot valves 7 96 are sequentially operated for sufficient time to open the valves 95 and cause pressurized air to flow through the headers 93 and the orifices therein into the venturis opening 87 to intermittently inflate the bags 27.

A rotatable cylindrical dump valve 104 at the lower end of powder separator 23 is similar to dump valve 47 described hereinbefore and is continuously rotated by a motor 106 to prevent accumulation of sufficient overspray powder in separator 23 to result in compaction of the powder which might result in obstruction of the recirculation system. Cylindrical dump valve 104 is disposed between opposed, elongated, arcuate-in-cross section depressions 107 in the valve housing walls and has a close fit therewith which permits fan 24 to develop a relatively large negative pressure in housing 86 and ducts 21 to draw the overspray resin particles from booth 11 into powder separator housing 86. At its lower end powder separator housing 86 has a relatively flexible, inverted conical sidewall 108 of steep slope to prevent accumulation of the overspray resin particles, and electromagnetic vibrator means 110, similar to vibrator 42 on powder supply hopper 39, flex and vibrate the sidewall 108 to prevent accumulation and compaction of the powder.

The duct 28 between powder separator housing 86- and recirculated screening bin 30 is of steeper slope than shown in FIG. 7 to assure that the overspray resin particles do not accumulate.

The dispensing means for new powder shown in FIG. 7 are variable in speed. Two bin level controls 57L and 57U of the dynamometer type aremounted at different heights on the sidewall of powder supply hopper 39 and have paddle wheels 114 rotatable within supply hopper 39 by motor means 1 16. The output terminals of upper bin level control 57U arecoupled to valve motor control 59, and the output terminal of lower bin control 57L is connected by a conductor 117 to the operating coil of a relay 118. When the new and recirculated overspray powder within supply hopper 39 is below a first predetermined amount, preferably three pounds, the paddle wheels 114L and 114U of both bin level controls 57U and 57L turn freely, no signal is provided to valve motor control 59, and control 59 energizes motor 52 through a resistance 120 to rotate dump valve 47 at a relatively high speed to deliver new powder to supply hopper 39. When the new and recirculated powder within hopper 39 reaches a level corresponding to said first predetermined amount, paddle wheel 1 14L of lower bin level control 57L is prevented from rotating by the powder, and control 57L provides an output to the operating coil of relay 118 which actuates its movable contact 122 relative to a pair of stationary contacts 123 connected to spaced taps on resistor 120 to thereby change the resistance in series with the field winding of motor 52 and slow down motor 52 and the speed at which dump valve 47 delivers new powder to supply hopper 39. When the new and recirculated resin powder reaches a level corresponding to a second predetermined amount which may result in compaction of the powder, preferably 5 pounds, paddle wheel 114U of upper bin control 57U can no longer rotate, and upper bin level control 57U provides an output signal to valve motor control 59 which opens the energizing circuit to motor 52 to thereby stop delivery of new powder to supply hopper 39 before a sufficient amount of powder can accumulate in supply hopper 39 to result in compaction of the powder. It will be appreciated that the disclosed new powder dispensing means also constitutes means for minimizing the amount of resin powder in the dielectric spray coating system.

FIGS. 9 and 10 schematically illustrate an embodiment of the invention which permits a still further reduction in the amount of resin powder in the electrostatic spray coating system and also further reduces the possibility of shutdowns caused by obstructions in the system. Coarse or agglomerated resin particles and ferromagnetic particles are removed from the new powder before it is introduced into the coating system. Elements of the system of FIGS. 9 and 10 similar to those of the embodiments of FIGS. 1-6 and FIGS. 7-8 are given the same reference numerals. The new powder hopper is eliminated in this embodiment, and new dry resin powder is conveyed from a shipping drum 124 by a flexible tubular conduit 125 enclosing a screw conveyor 128 rotatable within tubularconduit 125 by an electric motor 129. The feed inlet end of the screw conveyor conduit 125 is inserted into the new powder within drum 124, and the discharge end 131 thereof communicates with a duct 132 registering with one inlet opening into a chamber 134. The chamber 134 has a second inlet opening in an upper wall thereof communicating with the duct 28 from the powder separator housing 86 and also has an outlet opening at its lower end communicating with recirculated powder screening bin 30. The new resin powder delivered from drum 124 by screw conveyor conduit 125 into duct 132 when motor 129 is energized falls by gravity onto the grate type hopper magnet 32 which attracts any ferromagnetic particles therein and then onto the inclined vibrating screen 33 which blends the new and recirculated powder, and any agglomerated resin particles or debris mixed with the new powder do not pass through the screen 33 and slide down the vibrating screen 33 and fall into sight glass container 37.

The blended new and recirculated powder particles which pass through screen 33 fall into the cone-shaped powder supply hopper 39. The powder level detector 136 of this embodiment preferably has an elongated vertical vibrating finger, or rod probe 137 positioned within hopper 39 slightly above the bottom wall 140 thereof. The lower end of rod probe 137 preferably depends below that level 142 of powder (see FIG. 10) to which the resin particles fall due to gravity and the vibration of the walls of hopper 39 by the electromagnetic vibrators 42 but above the powder level 144 in alignment with the appertures 145 leading to the venturi chambers 60 in housings 61 into which apertures the powder is drawn by negative pressure. Level detector 136 may be mounted on a bracket 147 affixed to a sidewall of recirculated powder screening bin 30 and may be of the type sold under the trademark DYNATROL described in Bulletin No. DJ-69 of Automation Products, Inc. of Houston, Texas wherein an electric drive coil shown schematically at 138 drives rod probe 137 into self-sustained mechanical vibration at its natural resonant frequency when rod 137 is uncovered, and a pickup coil shown schematically at 139 is excited by the mechanical oscillations of the rod 137 and generates an alternating voltage signal when the level of powder in bin 39 is low and rod 137 is uncovered. This low powder level signal may operate a relay (not shown) in motor control 146 (illustrated in block form) which completes an energizing circuit to motor 129 to rotate conveyor screw 128 and thus deliver new powder from drum 124 into chamber 134.

When the powder within hopper 39 approaches level 142, for example, one inch to two inches above bottom wall 140, wherein the powder obstructs vibration of rod probe 137, dampening of the rod osicllations occurs, the magnitude of the rod oscillations is greatly reduced, and the output from the pickup coil 139 drops to such a low level that motor control 146 opens the energizing circuit tomotor 129 to stop delivery of new powder by screw conveyor 125 to chamber 134. This powder level detector 136 is very sensitive and permits operation of the powder system of the invention with only I or 2 pounds of resin powder in the entire system without the powder level falling to the point where portions of the articles are not covered by the resin.

In this embodiment, the length of the legs 148 of the support table 149 are shown to be of sufficient height so that the venturis 62 are raised above the level of the spray nozzles 17, thereby assuring that no U-shaped bends occur in the flexible hoses 70 between the venturis 62 and the spray nozzles 17 in which the resin powder particles can become compacted and cause obstructions.

The disclosed powder-air venturi construction eliminates clogging of the powder dispensing means as a result of electrostatic attraction of the powder particles and substantially eliminates shutdowns of the equipment as frequently occurred with prior art apparatus.

Whileonly a single embodiment of our invention has been illustrated and described, many modifications and variations thereof will be readily. apparent to those skilled in the art, and consequently it should be understood that we do not intend to be limited to the particular embodiment shown and described.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

ll. In an electrostatic spray system for coating an article with resin powder, in combination, a powder-air venturi housing, of insulating material having a hollow chamber and a vertically extending powder inlet port communicating with said chamber so that powder can be fed by gravity into said chamber and an outlet passage communicating with said chamber and transverse to said powder inlet port, and also having a restricted pressurized air inlet opening into said chamber aligned with said outlet passa e, an electrostatic spray nozzle, conduit means coup mg said outlet charged resin powder particles are not attracted to said housing.

2. In an electrostatic spray system in accordance with claim 1, and including a pressurized air supply nozzle having an axially extending restricted aperture therein communicating with said chamber and being aligned with said outlet passage and constituting said restricted pressurized air inlet opening into said chamber, said air supply nozzle being isolated from electrical ground and being selectively movable into and out of said chamber to positions wherein it partially obstructs said powder inlet port and thus controls the amount of powder being propelled from said chamber by the compressed air.

3. In an electrostatic spray system in accordance with claim 2, wherein said air supply nozzle is of insulating material. a

4. In an electrostatic spray system in accordance with claim 2, wherein said powder-air venturi housing has an air supply nozzle receiving compartment communicating with said chamber and being provided with internal threads and also has a pressurized air inlet port communicating with said compartment, and wherein said air supply nozzle has external threads engaging said internal threads and a reduced diameter portion with a radial aperture therein registering with said axially extending restricted pressurized air aperture and sealing means on said air supply nozzle spaced apart in an axial direction and being in sealing engagement with the walls of said compartment on opposite sides of said radial aperture and also on opposite sides of said air inlet port as said nozzle is moved into and out of said housing. 

1. In an electrostatic spray system for coating an article with resin powder, in combination, a powder-air venturi housing of insulating material having a hollow chamber and a vertically extending powder inlet port communicating with said chamber so that powder can be fed by gravity into said chamber and an outlet passage communicating with said chamber and transverse to said powder inlet port, and also having a restricted pressurized air inlet opening into said chamber aligned with said outlet passage, an electrostatic spray nozzle, conduit means coupling said outlet passage to said spray nozzle, a pressurized air supply, and means for connecting said pressurized air supply to said restricted air inlet opening to propel said powder fed through said powder inlet port out of said chamber and through said outlet passage and said conduit means and said spray nozzle, whereby electrostatically charged resin powder particles are not attracted to said housing.
 2. In an electrostatic spray system in accoRdance with claim 1, and including a pressurized air supply nozzle having an axially extending restricted aperture therein communicating with said chamber and being aligned with said outlet passage and constituting said restricted pressurized air inlet opening into said chamber, said air supply nozzle being isolated from electrical ground and being selectively movable into and out of said chamber to positions wherein it partially obstructs said powder inlet port and thus controls the amount of powder being propelled from said chamber by the compressed air.
 3. In an electrostatic spray system in accordance with claim 2, wherein said air supply nozzle is of insulating material.
 4. In an electrostatic spray system in accordance with claim 2, wherein said powder-air venturi housing has an air supply nozzle receiving compartment communicating with said chamber and being provided with internal threads and also has a pressurized air inlet port communicating with said compartment, and wherein said air supply nozzle has external threads engaging said internal threads and a reduced diameter portion with a radial aperture therein registering with said axially extending restricted pressurized air aperture and sealing means on said air supply nozzle spaced apart in an axial direction and being in sealing engagement with the walls of said compartment on opposite sides of said radial aperture and also on opposite sides of said air inlet port as said nozzle is moved into and out of said housing. 